1. Field of the Invention
The present invention relates to a liquid crystal panel and a method for manufacturing the same. In particular, the present invention relates to a liquid crystal panel which is suitable for use as a liquid crystal light valve for a projection display (liquid crystal projector) and a method for manufacturing the same.
2. Description of the Related Art
A liquid crystal panel used as a liquid crystal light valve for a liquid crystal display device and even for a liquid crystal projector has a liquid crystal layer sealed between a pair of substrates provided with alignment films thereon. In manufacturing such a liquid crystal panel, an alignment film is formed by rubbing a polyimide or polyamide resin film.
Rubbing, however, causes problems such as formation of scratches and foreign substances on a surface of the resin film (alignment film). In addition, the resin film may not be rubbed uniformly over the entire surface. Furthermore, the resin alignment film easily deteriorates with light. For a liquid crystal projector, its liquid crystal light valve (liquid crystal panel) is irradiated with intense light (incident light) from a light source. Since the intensity of the incident light is increasing nowadays, a resin alignment film is more likely to deteriorate to such a degree as to cause bubbles.
In viewing this, a method for forming an alignment film of an inorganic material such as silicon oxide was proposed and is now used in practice. This method is referred to as oblique deposition in which vapor of an inorganic material such as silicon oxide is supplied at a predetermined oblique angle relative to the normal of a substrate to form an alignment film composed of deposited molecules (molecules of the inorganic material) oriented in the same direction on a surface of the substrate. An alignment film formed by this non-rubbing oblique deposition is free of foreign substances, scratches, or poor alignment. Another advantage of oblique deposition is that the inorganic material used is light resistant to such a level that deterioration of the film is negligible.
Oblique deposition, however, exhibits the following disadvantage when the alignment film formed is used in a liquid crystal panel in TN mode.
When a voltage is applied to drive a liquid crystal panel, the liquid crystal molecules must be preliminary tilted at a predetermined angle (pre-tilt angle) relative to the substrate, in order to align the rising direction of the liquid crystal molecules.
Taking into consideration the fact that a pre-tilt angle from about 5xc2x0 to 12xc2x0 is ideal for a TN-mode liquid crystal panel, an alignment film formed by rubbing is more advantageous because it provides a pre-tilt angle from about 2xc2x0 to 7xc2x0. On the other hand, an alignment film formed by oblique deposition of silicon oxide may provide a pre-tilt angle as high as 20xc2x0 to 30xc2x0 or 0xc2x0 (no tilting) depending on the deposition angle. Thus, a TN-mode liquid crystal panel including alignment films formed by oblique deposition of an inorganic material cannot satisfactorily align the rising directions of liquid crystal molecules at a time of voltage application, leading to display defects accordingly.
In order to solve the foregoing problems, a liquid crystal panel according to the present invention has a liquid crystal layer sealed between a pair of substrates having alignment films thereon. Each of the alignment films is composed of a first deposited layer and a second deposited layer on top of the first deposited layer by oblique deposition of an inorganic material, the deposited molecules of the first deposited layer being aligned substantially perpendicular to the orientation of the deposited molecules of the second deposited layer. In this liquid crystal panel, the first deposited layer has the deposited molecules aligned perpendicular to the orientation of the liquid crystal molecules, and may be formed by oblique deposition that supplies the deposited molecules at a predetermined angle (deposition angle) of, for example, about 60xc2x0 relative to the normal of the substrate surface. The second deposited layer has the deposited molecules aligned parallel to the orientation of the liquid crystal molecules, and may be formed by oblique deposition that supplies the deposited molecules at a predetermined angle (deposition angle) of about 85xc2x0 relative to the normal of the substrate surface.
According to the present invention, a method for manufacturing such a liquid crystal panel includes the steps of forming a first deposited layer on the substrates by oblique deposition and forming a second layer on top of the first layer also by oblique deposition. In the second step above, the deposition molecules are supplied onto the surfaces of the substrates from a direction shifted by about 90xc2x0 from the oblique deposition direction in the first step.
In the liquid crystal panel and the method for manufacturing the liquid crystal panel according to the present invention, each of the alignment films is formed of a first deposited layer, whose deposited molecules are aligned at an angle of 90xc2x0 to the orientation of the liquid crystal molecules, and a second deposited layer stacked on top of the first deposited layer such that the deposited molecules of the first deposited layer are aligned substantially perpendicular to the orientation of the deposited molecules of the second deposited layer. In such an alignment film, a desirable pre-tilt angle of liquid crystal molecules is achieved by adjusting the thickness of the second deposited layer relative to the thickness of the first deposited layer. FIG. 1 shows how pre-tilt angles of liquid crystal molecules change depending on the thickness of the second deposited layer (deposition angle of 85xc2x0) with respect to the first deposited layer with a thickness of about 40 nm (deposition angle of 60xc2x0). As shown in FIG. 1, the pre-tilt angles of liquid crystal molecules are substantially proportional to the thickness of the second deposited layer. Based on the graphs shown in FIG. 1, the second deposited layer preferably has a thickness of about 0.1 nm to 0.5 nm to achieve a pre-tilt angle of about 5xc2x0 to 12xc2x0, which is ideal for a TN-mode liquid crystal panel.
Thus, the liquid crystal panel and the method for manufacturing the liquid crystal panel according to the present invention are advantageous in that the liquid crystal panel has alignment films of inorganic material resistant to light and provides a pre-tilt angle suitable for a TN-mode liquid crystal panel, for example, by properly controlling the thickness of the two deposited layers constituting each of the alignment films.
As a result, the liquid crystal panel according to the present invention is superior in long-term reliability, prevents display defects at the time of voltage application, and provides a quick response.